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9. Binary Tree Inorder Traversal

easyAsked at Lyft

Return the inorder traversal of a binary tree.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given the root of a binary tree, return the inorder traversal of its nodes' values (left, root, right). Solve it both recursively and iteratively.

Constraints

  • 0 <= number of nodes <= 100
  • -100 <= node value <= 100

Examples

Example 1

Input
root = [1,null,2,3]
Output
[1,3,2]

Example 2

Input
root = []
Output
[]

Approaches

1. Recursive

Visit left subtree, current, then right subtree.

Time
O(n)
Space
O(n)
function dfs(n,out){if(!n)return; dfs(n.left,out); out.push(n.val); dfs(n.right,out);}

Tradeoff:

2. Iterative with stack

Push left chain, pop and visit, then dive into right. Explicit stack avoids recursion limits.

Time
O(n)
Space
O(h)
function inorderTraversal(root) {
  const out = [];
  const stack = [];
  let cur = root;
  while (cur || stack.length) {
    while (cur) { stack.push(cur); cur = cur.left; }
    cur = stack.pop();
    out.push(cur.val);
    cur = cur.right;
  }
  return out;
}

Tradeoff:

Lyft-specific tips

Lyft expects the iterative form when interviewing senior candidates because their map service avoids deep recursion on geohash trees in production.

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